Pyridine-linked covalent triazine frameworks with bidirectional electron donor-acceptor for efficient organic pollution removal

Dual functional covalent triazine framework-TiO2 S-scheme heterojunction for efficient sequestration of ciprofloxacin: Mechanism and degradation products

The development of adsorbent and/or photocatalysts based on covalent triazine frameworks (CTF) is fascinating research due to their structural properties, functional groups, and active sites. Herein, a CTF-TiO2 heterojunction was synthesized by modifying CTF sheets with TiO2 particles through wet impregnation technique and adsorptive and photocatalytic activities determined for ciprofloxacin (CIP) removal. Comprehensive characterisation of the composites revealed suitable properties of the composites, such as sandwich-like CTF-TiO2 morphology, improved thermal stability, and better heteroatom effect (HAE). The adsorption capacity of CTF-TiO2-1 (CT-1) and CTF-TiO2-2 (CT-2) reached 30.30 mg g-1 and 13.61 mg g-1, respectively. Meanwhile, the CT-2/H2O2 system, compared to all other materials, achieved a better degradation efficiency of 90.7 % within 40 min compared to 77.5 % observed in using only CT-2 for 120 min. In addition, scavenging results suggested that e- and h+ was crucial for the effective degradation of CIP. Identification of the degradation product of CIP suggests hydroxylation, decarboxylation, and opening of the quinolone and piperazine ring as possible degradation pathways. The mineralization of CIP was 90.93 % for the CT-2/H2O2 system and its stability maintained for four cycles. The outstanding performance of CT-2 is attributed to its enhanced band gap energy of 2.86 eV, and reduced recombination rate of photogenerated electrons and holes. These results prove these materials are efficient adsorbent/photocatalyst in CIP removal from solution.

Linkage Microenvironment Modulation in Triazine-Based Covalent Organic Frameworks for Enhanced Photocatalytic Hydrogen Peroxide Production

Covalent organic frameworks (COFs), known for the precise tunability of molecular structures, hold significant promise for photocatalytic hydrogen peroxide (H2O2) production. Herein, by systematically altering the quinoline (QN) linkages in triazine (TA)-based COFs via the multi-component reactions, six R-QN-TA-COFs are synthesized with identical skeletons but different substituents. The fine-tuning of the optoelectronic properties and local microenvironment of COFs is allowed, thereby optimizing charge separation and improving interactions with dissolved oxygen. Consequently, MeO-QN-TA-COF is customized to achieve an impressive rate of H2O2 production up to 7384 µmol g⁻1 h⁻1 under an air atmosphere in water without any sacrificial agents, surpassing most of the reported COF photocatalysts. Its high stability is demonstrated through five consecutive recycling experiments and the characterization of the recovered COF. The reaction mechanism for the H2O2 production is further investigated using a suite of quenching experiments, in situ spectroscopic analysis, and theoretical calculations. The enhanced photocatalytic H2O2 production over MeO-QN-TA-COF is through 2e⁻ oxygen reduction reaction and water oxidation reaction pathways. Overall, the crucial role of linkage microenvironment modulation in the design of COFs for solar-driven effective photocatalytic H2O2 production.

Insights into excitonic behavior in single-atom covalent organic frameworks for efficient photo-Fenton-like pollutant degradation

The generation of radicals through photo-Fenton-like reactions demonstrates significant potential for remediating emerging organic contaminants (EOCs) in complex aqueous environments. However, the excitonic effect, induced by Coulomb interactions between photoexcited electrons and holes, reduces carrier utilization efficiency in these systems. In this study, we develop Cu single-atom-loaded covalent organic frameworks (CuSA/COFs) as models to modulate excitonic effects. Temperature-dependent photoluminescence and ultrafast transient absorption spectra reveal that incorporating acenaphthene units into the linker (CuSA/Ace-COF) significantly reduces exciton binding energy (Eb). This modification not only enhances peroxymonosulfate adsorption at Cu active sites but also facilitates rapid electron transfer and promotes selective hydroxyl radical generation. Compared to CuSA/Obq-COF (Eb = 25.6 meV), CuSA/Ace-COF (Eb = 12.2 meV) shows a 39.5-fold increase in the pseudo-first-order rate constant for sulfamethoxazole degradation (0.434 min-1). This work provides insights into modulating excitonic behavior in single-atom catalysts via linker engineering for EOCs degradation.

Covalent triazine frameworks as particle electrode for three-dimensional photoelectrocatalytic degradation of oxytetracycline: Synergy effects, pathway, and mechanism

Photoelectrocatalysis has been widely employed for degrading antibiotics due to its high efficiency. However, the application is significantly impeded by the rapid recombination of photogenerated charge carriers and the limited surface areas of photoelectrodes. In the study, high crystallinity covalent triazine frameworks were fabricated at low temperature of 150 °C and firstly used as particle photoelectrode in the three-dimensional photoelectrochemical reactor to degrade oxytetracycline (OTC). SEM, TEM, XRD, XPS, and FT-IR confirmed the successful synthesis of high crystallinity covalent triazine frameworks. Compared to CTF-120 (71.2%) and CTF-180 (46.9%), CTF-150 exhibited excellent OTC removal. Electrochemical impedance, UV-vis absorption spectra, and Mott-Schottky tests showed that CTF-150 demonstrated more wide light absorption range of 501 nm and narrow bandgap of 2.52 eV, and smaller Rct value under illumination, in comparing to CTF-120 and CTF-180. When the initial concentration of OTC was 50 mg L-1, the 86.2% of OTC removal and 62.7% of mineralization were obtained under light irradiation (λ > 420 nm), current of 10 mA, pH of 6.4, electrolyte of 0.1 M Na2SO4. The synergy effect between photocatalytic and electrocatalytic processes of CTF-150 not only enhanced by 38.5% current efficiency but also reduced energy consumption to 1.90 kWh m-3. CTF-150 had a wide range of acid-base application and displayed resistance on coexisting ions. Electron spin resonance detection, quenching experiments, and probe experiments illustrated that h+, •O2-, 1O2, and •OH contributed to the degradation of OTC and the generation pathways of •O2-, 1O2, and •OH were verified. Moreover, •O2-, 1O2, and h+ were the main reactive species responsible for OTC removal, while 1O2 was for OTC mineralization. Based on high-performance liquid chromatography-tandem mass spectrometry detection, OTC with benzene ring was decomposed to opening ring products. The acute toxicity, developmental toxicity, bioaccumulation factor and mutagenicity of OTC and its intermediates using T.E.S.T. showed the toxicity of 82.35% degradation products decreased.

Vapor-Solid Interface Synthesis of Highly Crystalline Covalent Triazine Frameworks for Use as Efficient Photocatalysts

Harsh synthetic conditions for crystalline covalent triazine frameworks (CTFs) and associated limitations on structural diversities impede not only further development of functional CTFs, but also practical large-scale synthesis. Herein, a mild and universal vapor-solid interface synthesis strategy is developed for highly crystalline CTFs employing trifluoromethanesulfonic acid vapor as catalysts. A series of highly ordered simple and functional CTFs (CTF-TJUs) can be facilely produced. In particular, the porphyrin-involved functional CTF (CTF-TJU-Por1) with high crystallinity is synthesized for the first time via this universal approach. The mechanism of vapor-catalyzed trimerization of nitrile monomers is thoroughly investigated through semi in situ characterizations. As a proof of concept, the photocatalytic performance of synthesized CTFs for water splitting is evaluated. CTF-TJU-133 exhibits significantly greater photocatalytic rates for hydrogen (4.35 µmol h-1) and oxygen (2.18 µmol h-1) evolutions during overall water splitting under visible light irradiations compared to other CTF-TJUs, representing one of the highest values among reported CTF photocatalysts. Further studies reveal that enhanced photocatalytic performance of CTF-TJU-133 results from optimized band structure, extended visible-light absorption, and high carrier separation efficiency. This study provides a promising strategy to synthesize various simple and functional CTFs, which significantly enriched diversities of CTF family for different application purposes.

Donor-acceptor type COFs with multiple fluorine groups as electron storage units to promote antimicrobial performance

The novel donor-acceptor (D-A) type covalent organic frameworks TATF-COF and TATP-COF, with multiple fluorine groups as electron storage units, were successfully constructed to achieve efficient charge transfer and photocatalytic activity for antibacterial photocatalytic therapy. Fluorine, the most electronegative element, was utilized as an electron-withdrawing substituent for the acceptor, which could unite the donor unit together and efficiently improve the charge transfer from the donor to acceptor. The unique D-A structures of TATF-COF and TATP-COF ensure that they have narrow band gaps, strong photocurrent responses, long fluorescence lifetimes, and good capacity to generate reactive oxygen species (ROS) to realize good antibacterial activity. Meanwhile, the inclusion of multiple hydrophilic fluorine groups means that TATF-COF and TATP-COF are highly water dispersible, which is also beneficial in terms of promoting the generation of adequate quantities of ROS. Hence, in view of their excellent photoelectric properties and good water dispersibility, further investigations were performed, and excellent antibacterial activities in vitro against both gram-negative and gram-positive bacteria were demonstrated for TATF-COF and TATP-COF. In addition, we also showed that they can function as effective antibacterial dental materials.

Efficient integration of covalent triazine frameworks (CTFs) for augmented photocatalytic efficacy: A review of synthesis, strategies, and applications

Heterogeneous photocatalysis emerges as an exceptionally appealing technological avenue for the direct capture, conversion, and storage of renewable solar energy, facilitating the generation of sustainable and ecologically benign solar fuels and a spectrum of other pertinent applications. Heterogeneous nanocomposites, incorporating Covalent Triazine Frameworks (CTFs), exhibit a wide-ranging spectrum of light absorption, well-suited electronic band structures, rapid charge carrier mobility, ample resource availability, commendable chemical robustness, and straightforward synthetic routes. These attributes collectively position them as highly promising photocatalysts with applicability in diverse fields, including but not limited to the production of photocatalytic solar fuels and the decomposition of environmental contaminants. As the field of photocatalysis through the hybridization of CTFs undergoes rapid expansion, there is a pressing and substantive need for a systematic retrospective analysis and forward-looking evaluation to elucidate pathways for enhancing performance. This comprehensive review commences by directing attention to diverse synthetic methodologies for the creation of composite materials. And then it delves into a thorough exploration of strategies geared towards augmenting performance, encompassing the introduction of electron donor-acceptor (D-A) units, heteroatom doping, defect Engineering, architecture of Heterojunction and optimization of morphology. Following this, it systematically elucidates applications primarily centered around the efficient generation of photocatalytic hydrogen, reduction of carbon dioxide through photocatalysis, and the degradation of organic pollutants. Ultimately, the discourse turns towards unresolved challenges and the prospects for further advancement, offering valuable guidance for the potent harnessing of CTFs in high-efficiency photocatalytic processes.

Efficient Proton Transfer and Charge Separation within Covalent Organic Frameworks via Hydrogen-Bonding Network to Boost H2O2 Photosynthesis

Photocatalytic synthesis based on the oxygen reduction reaction (ORR) has shown great promise for H2O2 production. However, the low activity and selectivity of 2e- ORR result in a fairly low efficiency of H2O2 production. Herein, we propose a strategy to enhance the proton-coupled electron transfer (PCET) process in covalent organic frameworks (COFs), thereby significantly boosting H2O2 photosynthesis. We demonstrated that the construction of a hydrogen-bonding network, achieved by anchoring the H3PO4 molecular network on COF nanochannels, can greatly improve both proton conductivity and photogenerated charge separation efficiency of COFs. Thus, COF@H3PO4 exhibited superior photocatalytic performance in generating H2O2 without sacrificial agents, with a solar-to-chemical conversion efficiency as high as 0.69%. Results indicated that a much more localized spatial distribution of energy band charge density on COF@H3PO4 led to efficient charge separation, and the small energy barrier of the rate-limiting step from *OOH to H2O2 endowed COF@H3PO4 with higher 2e- ORR selectivity.

Fabrication of imidazoline-linked cationic covalent triazine framework for enrichment of environmental estrogens

Imidazoline-linked cationic covalent triazine framework (IM-iCTF) was facilely prepared through the Debus-Radziszewski reaction, involving 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline, formaldehyde and methylglyoxal. The IM-iCTF was applied as a sorbent for cartridge solid-phase extraction (SPE). It provided good adsorption performance for estrogen and estrogen mimics including bisphenol F, bisphenol A, 7β-estradiol, bisphenol B and estrone. The adsorption isotherm, adsorption kinetic model, thermodynamic calculations and adsorption mechanism were investigated to reveal the adsorption behavior. The IM-iCTF was employed for the extraction of the estrogens and estrogen mimics from water, fish and shrimp (fish and shrimp samples were extracted with acetonitrile before the SPE). The analytes were then determined by high-performance liquid chromatography with diode array detection. The limits of detection were 0.008-0.05 ng mL-1 for water, 0.015-0.11 μg g-1 for fish, and 0.012-0.10 μg g-1 for shrimp samples. This research not only offers a new approach to construct cationic covalent triazine framework, but also provides a reliable strategy for the adsorption/enrichment trace level of organic pollutants.

Unveiling the metallic size effect on O2 adsorption and activation for enhanced electro-Fenton degradation of aromatic compounds

Metal-atom-modified nitrogen-doped carbon materials (M-N-C) have emerged as promising candidates for electro-Fenton degradation of pollutants. Nonetheless, a comprehensive exploration of size-dependent M-N-C catalysts in the electro-Fenton process remains limited, posing challenges in designing surface-anchored metal species with precise sizes. Herein, a heterogeneous-homogeneous coupled electro-Fenton (HHC-EF) system was designed and various M-N-C catalysts anchored with Co single atoms (CoSA-N-C), Co clusters (CoAC-N-C), and Co nanoparticles (CoNP-N-C) were successfully synthesized and employed in an HHC-EF system. Intriguingly, CoAC-N-C achieved outstanding removal efficiencies of 99.9% for BPA and RhB within 10 and 15 min, respectively, with the fastest reaction kinetics (0.70 min-1 for BPA and 0.34 min-1 for RhB). Electron spin resonance and trapping experiments revealed that·OH played a crucial role in the HHC-EF process. Moreover, experiments and theoretical calculations revealed that the unique metallic size effect facilitate the in-situ electro-generation of H2O2. Specifically, the atomic interaction between neighboring Co atoms in clusters enhanced O2 adsorption and activation by strengthening the Co-N bond and transforming O2 adsorption configuration to the Yeager-type. This study provides valuable insights that could inspire the size-oriented metal-based catalyst design from the perspective of the potential atomic distance effect.

Well-constructed a water stable Cu-BTC@TpPa-1 binary composite with excellent capture ability toward malachite green

Adsorptive removal of dyes (e.g., malachite green (MG)) from wastewater using commercially available adsorbents is not significantly efficient. Metal-organic frameworks (MOFs) such as Cu-BTC is considered as an excellent adsorbent in adsorption-separation filed. However, the water instability of Cu-BTC restricts its potential utilization in dye wastewater purification. In this paper, we have developed a novel metal/covalent-organic frameworks (Cu-BTC@TpPa-1) binary composite by solvothermal method. This composite serves as a multifunctional platform for the effective removal of MG from water. This Cu-BTC@TpPa-1 obviously keeps structural integrity soaked in water for 7 days. And its heat resistant performance can achieve 360 °C because of the TpPa-1 protection, which is outdistance to that of Cu-BTC. The adsorbed capacity of MG over Cu-BTC@TpPa-1 is exceptionally high, with an uptake of up to 64.12 mg/g, which is superior compared to previous adsorbents, highlighting its superior adsorption capabilities. The adsorptive performance was controlled by the associative effects of Cu-BTC and TpPa-1 with an association effect of π-complexation and electrostatic attraction. The Cu-BTC@TpPa-1 might be a prospective adsorbent for MG capture from industrial wastewater.

Insights into dual effect of missing linker-cluster domain defects for photocatalytic 2e- ORR: Radical reaction and electron behavior

Photocatalytic 2e^- ORR paves a promising avenue for hydrogen peroxide (H₂O₂) production. However, the obscure structure-activity relationship between a specific structure and photocatalytic 2e^- ORR restricts the understanding of its intrinsic mechanism. In this work, Metal-Organic Frameworks (MOFs) with missing linker-cluster domain (MLCD) defects were employed as a model to shed new light on the effect of MLCD defect on photocatalytic 2e^- ORR, which mainly focused on the radical reaction and electron behavior. Experiments and theoretical calculations revealed that incorporating MLCD defects significantly lowered the contribution rate of reactive superoxide radical (·O₂^-). Meanwhile, the retarded interfacial charge transfer via "Ti-O″ bridge was caused by the undesirable electron dissipation and augmented orbital-limited resistance induced by electron spin polarization. Therefore, the photocatalytic 2e^- ORR activity was decreased to 30% of its origin by construction MLCD defects. This study provides insights into the internal mechanism of photocatalytic 2e^- ORR for designing and optimizing excellent defective nanomaterials.